1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-rejecting portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) describe an ablative recording system that uses low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from the diode to the printing surface and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer).
U.S. Ser. Nos. 08/700,287 and 08/756,267, the entire disclosures of which are hereby incorporated by reference, describe a variety of lithographic plate configurations for use with such imaging apparatus. In general, the plate constructions include an inorganic layer (i.e., a metal, combination of metals, or a metal/non-metal compound) situated on an organic polymeric layer. The inorganic layer ablates in response to imaging (e.g., infrared, or "IR") radiation. In one approach, the inorganic layer represents the topmost surface of the plate and accepts fountain solution, while the underlying polymeric layer accepts ink. In another approach, the inorganic layer serves only a radiation-absorption (rather than a lithographic) function, with the underlying layer accepting ink and an overlying layer either rejecting ink or accepting fountain solution. Ablation of the inorganic layer by an imaging pulse generally weakens the topmost layer as well, and this, combined with disruption of its anchorage (due to disappearance of the ablated inorganic layer), renders the topmost layer easily removable in a post-imaging cleaning step. With either of these two approaches, application of an imaging pulse to a point on the plate ultimately creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of unexposed areas, the pattern of such spots forming a lithographic plate image.
These types of plates can pose manufacturing challenges, as well as performance limitations, owing to the abrupt transition between an inorganic layer and an organic, polymeric layer. The divergent physical and chemical characteristics of such distinct layers can compromise their anchorage to one another--a critical performance requirement--as well as the durability of the inorganic layer. For example, because inorganic and organic materials typically have very different coefficients of thermal expansion and elastic moduli, even perfectly adhered inorganic layers may undergo failure (e.g., fracturing) due to temperature variations or the stress of plate manipulation and use. The different responses of two adjacent layers to an external condition can easily cause damage that would not occur in either layer by itself.
To improve interlayer anchorage, polymeric layers may be selected (or applied as intermediate coatings) based on chemical compatibility with inorganic material. A polymeric layer may also be pretreated (e.g., through plasma exposure) to modify the surface for greater interfacial compatibility with a subsequently applied inorganic layer. These approaches, however, have limited utility in addressing the effects of transition between fundamentally different materials.